A prior art example of a general small printer using a line type thermal head will be explained based on a section view shown in FIG. 6.
In the figure, the prior art printer comprises a platen 5 provided for rotation with respect to a frame 1, a thermal head 6 pressed to the platen 5, a platen spring 19 for pressing the thermal head 6 resiliently to the platen 5, a platen spring receiving plate 20 for receiving resilience of the platen spring 19, a head-up lever 10 having a cam 10a for releasing the thermal head 6 from the platen 5 during maintenance for exchanging sheets of paper or a jammed paper, a head-up pin 8 provided on the thermal head 6 for engaging with the cam 10a to release the thermal head 6 from the platen 5, a thermal head pivot 17 which is a center axis of rotation of the thermal head 6, and other components.
In the small printer using the line thermal head, it is necessary to press and contact the thermal head to the platen with homogeneous pressure across the whole printing range from the aspect of printing quality and the like.
Accordingly, the prior art printer constructed as described above has been adapted such that one coil spring is disposed as the platen spring in a manner capable of freely rocking centering on a fulcrum so that its resilience acts homogeneously between the platen and the thermal head, or such that a plurality of coil springs each having homogeneous resilience are disposed at adequate points on the back of the thermal head to eliminate the unevenness of contact of the platen and the thermal head.
As a result, although the unevenness of contact of the platen and the thermal head has been improved, there has been a problem that the coil spring is repeatedly stretched and compressed every time when the head is opened and closed, thus causing a degradation of the spring such as settling as a result, because the thermal head has an opening/closing mechanism for maintenance with respect to the platen and the coil spring is designed so that it exerts an adequate resilience when the thermal head abuts with the platen. In case of the printer using the plurality of coil springs in particular, the resilience may vary per each coil spring, thus changing the pressing stress between the platen and the thermal head partially. Accordingly, there have been such problems with conventional line printers that the printing quality is degraded and failure of the printer itself may occur. There has been also another problem that because the repulsive resilience of the coil spring becomes large when it is compressed during head-up and the mechanical strength of the platen spring receiving plate for receiving such large resilience has to be increased, it has been difficult to design such printer from the aspect of its structure and strength.
Further, because the head-up operation carried out by means of the head-up lever has been carried out by abutting the head-up cam only to one side of the head-up pin and resisting to the resilience of the coil spring in general, there has been a problem that the other end of the thermal head which does not engage directly with the head-up cam has a large inclination and it becomes difficult to provide proper maintenance such as correcting a paper jam in a printer using a line thermal head having a wider width. There have been also other problems that the use of the coil spring which is cumbersome to assemble lowers the assembly efficiency of the printer and complicates the structure around the head of the printer, thus increasing the cost of the printer.
Accordingly, it is an object of the present invention to improve the printing quality further of a printer as well as to realize a printer having a more simplified structure and to improve the assembly efficiency thereof.